The official gazette of Japanese Patent Application Laid Open (JP-A) No. 2002-187508 discloses a hollow double wall structure for the absorption of impact energies to vehicles such as automobiles. This structure is produced by blow molding of thermoplastics, forming recessed ribs from the front surface wall and the rear surface wall with the top end parts thereof bonded with each other so as to be integrated for improving the energy absorbing property. Moreover, the official gazette of Japanese Patent No. 3,313,999 discloses an energy absorbing member made of a polypropylene resin having a 5,000 kg/cm2 to 25,000 kg/cm2 flexural modulus. Furthermore, the official gazette of Japanese Patent Application Laid Open (JP-A) No. 2002-201322 discloses one made of a polypropylene resin composition including a polypropylene component and a copolymer component of a propylene and an ethylene. According to the energy absorbing member disclosed in the above-mentioned official gazette of Japanese Patent, a sufficient shock absorbing property can be obtained by forming an interlocking rib for integrally linking a plurality of recessed ribs, or the like as shown in the official gazette of Japanese Patent Application Laid Open (JP-A) No. 2002-187508. However, it has been considered important that enhanced shock absorbing property particularly at low temperature is obtained. Regarding the energy absorbing member made of a polypropylene resin having a 5,000 kg/cm2 to 25,000 kg/cm2 flexural modulus, disclosed in the official gazette of Japanese Patent No. 3,313,999, it was afterwards discovered that the stress change at the time of energy absorption at 60° C. to −15° C. is large and the energy absorbing performance is effected by the outside air temperature. That is, with the load stress at a 50% compression distortion of a polypropylene energy absorbing member provided as the reference at an ordinary temperature, the stress change ratio is 21% in the case the outside air temperature is −15° C., and the stress change ratio is −34% in the case the outside air temperature is 60° C.
Since the stress change ratio of the energy absorbing member is required to be kept within ±10% at the temperature from −15° C. to 60° C., with respect to the compression distortion at ordinary temperature, the impact absorbing member formed of a polypropylene resin having a 5,000 kg/cm2 to 25,000 kg/cm2 flexural modulus disclosed in the official gazette of Japanese Patent No. 3,313,999 can not provide the required level of performance.